Preheat the oven to 425F (220C) and line a baking sheet with parchment paper.
Unfold the puff pastry sheet on a work surface. Using a pizza cutter or sharp knife, cut the dough into 8 triangles and separate them. Place them in the fridge while preparing the almond filling.
In a bowl, add the butter, sugar, salt, and vanilla extract. Whisk until the mixture is well combined and fluffy.
Add half of the whisked egg (reserve the other half for the egg wash) and whisk until fully incorporated.
Add the almond flour and all-purpose flour, then fold them in using a rubber spatula.
Take one puff pastry triangle out of the fridge and place it on a work surface. Spread approximately 1 tablespoon of almond filling evenly over the triangle.
Roll the croissant from the short side of the triangle towards the tip. The croissant should be rolled tightly but not so much that the layers are compressed.
Place the croissant on the baking sheet with the pointed tip tucked underneath. Gently press on top of the croissant to secure its shape. Repeat with the remaining triangles.
Brush the croissants with the reserved whisked egg and sprinkle the tops with sliced almonds.
Bake for 15-20 minutes or until golden brown on top.
Remove from the oven and let them cool before dusting with powdered sugar. Serve and enjoy!